1. Field of the Invention
This invention relates to a liquid crystal display device, and particularly to an active matrix type liquid crystal display device.
2. Description of Related Art
Conventionally an amorphous silicon thin film transistor (a-Si TFT) of channel etching type has been known as a switching element for an active matrix type liquid crystal display device.
FIG. 1 is a plan view of a conventional TFT array for liquid crystal display device. The drawing shows a portion of unit pixel. FIG. 2 is a sectional view of a TFT part of FIG. 1, and FIGS. 3A and 3B show terminal parts of FIG. 1 and FIG. 3A is a sectional view of a gate terminal electrode part and FIG. 3B is a sectional view of a data terminal electrode part.
As shown in FIG. 1, an a-Si TFT 1 is provided every pixel of an intersection of an XY matrix, and has a gate electrode 2, a source electrode 3 and a drain electrode 4 oppositely placed over the gate electrode 2. The gate electrode 2 is connected to a gate line 2a. The source electrode 3 is connected to a data line 3a. The drain electrode 4 is connected to a pixel electrode 6 via a contact through-hole 5.
As shown in FIG. 2, the gate electrode 2 formed on a transparent insulating substrate 7a is covered with a gate insulating film 7b and further a semiconductor layer 8 is formed in a position on the gate insulating film 7b superimposed on the gate electrode 2. The source electrode and the drain electrode 4 separated over the center of the semiconductor layer 8 are connected to the semiconductor layer 8 through an ohmic contact layer 9.
The ohmic contact layer 9 is etched and removed between the source electrode 3 and the drain electrode 4 is formed between the source electrode 3 and the semiconductor layer 8 and formed between the drain electrode 4 and the semiconductor layer 8. Further, a passivation film 7c is formed so as to cover the source electrode 3, the drain electrode 4, the ohmic contact layer 9 and the semiconductor layer 8. A transparent conductive film acting as the pixel electrode 6 is connected to the drain electrode 4 via the contact through-hole 5 through the passivation film 7c. 
A switching signal is inputted to the gate electrode 2 of this a-Si TFT 1 through the gate line 2a and a video signal voltage is inputted to the source electrode 3 of this a-Si TFT 1 through the data line 3a, respectively and the video signal voltage is written to the pixel electrode 6.
As shown in FIG. 3A, in the gate terminal electrode part, a gate electrode pad 6a made of transparent conductive layer connected to the gate electrode 2 on the transparent insulating substrate 7a is exposed on the gate insulating film 7b and the passivation film 7c to form a gate terminal electrode 2b. As shown in FIG. 3B, in the data terminal electrode part, a drain electrode pad 6b made of transparent conductive layer connected to a data line on the gate insulating film 7b is exposed on the passivation film 7c to form a data terminal electrode 4a. 
FIGS. 4A-4E are step diagrams showing a manufacturing method of the TFT array of FIG. 1 for the TFT part. As shown in FIG. 4A, first, a conductive layer made of aluminum (Al), molybdenum (Mo), chromium (Cr), etc. is deposited on a transparent insulating substrate 7a made of glass etc. with a thickness from about 100 to 400 nm by sputtering method.
Thereafter, first patterning for forming gate line (not shown), a gate electrode 2 and a gate terminal electrode (not shown) by a photolithography step is performed (see FIG. 4A). This gate terminal electrode (see FIG. 3A) is connected to an external signal processing substrate for display.
Next, as shown in FIG. 4B, a gate insulating film 7b made of a silicon nitride film etc., a semiconductor layer 8 made of amorphous silicon and an ohmic contact layer 9 made of n+ amorphous silicon are successively stacked with thicknesses of the order of 400 nm, 300 nm, 50 nm, respectively, by plasma CVD. After the stacking, second photolithography step for patterning the semiconductor layer 8 and the ohmic contact layer 9 in a batch is performed.
Then, as shown in FIG. 4C a conductive layer made of Mo, Cr, etc. is deposited with a thickness from about 100 to 200 nm by sputtering method so as to cover the gate insulating film 7b and the ohmic contact layer 9. After the deposition, third photolithography step for forming a source electrode 3, a drain electrode 4 and a data line 3a is performed.
Along with this third photolithography step, the unnecessary ohmic contact layer 9 other than the lower portion of the source electrode 3 and the drain electrode 4 acting as a channel part of an a-Si TFT 1 is removed (see FIG. 4C).
Then, as shown in FIG. 4D, a passivation film 7c made of an inorganic insulating layer such as a silicon nitride film is formed with a thickness from about 100 to 200 nm by a plasma CVD so as to cover a back channel of the a-Si TFT 1, the source electrode 3, the data line 3a, the drain electrode 4 and the data terminal electrode (not shown).
After the film formation, fourth photolithography step for forming a contact through-hole 5 for making contact with the drain electrode 4 and a pixel electrode 6 and removing the unnecessary passivation film 7c on the data terminal electrode part (not shown) and the unnecessary gate insulating film 7b and passivation film 7c on the gate terminal electrode (not shown) is performed.
Further, as shown in FIG. 4E, a transparent conductive film acting as the pixel electrode 6 is formed by sputtering method and fifth photolithography step is performed.
In this manner, a TFT array is manufactured via the above-mentioned five photolithography steps (see FIGS. 4A to 4E). A liquid crystal display device is formed by sandwiching liquid crystal between two substrates in which this TFT array substrate is combined with another substrate for providing a color filter layer and a common electrode.
With respect to this conventional TFT array, development of a technique of improving performance of the liquid crystal display device by providing an organic insulating layer on the TFT array has been activated in recent years.
For example, a technique (organic interlayer separation technique) of controlling disclination of liquid crystal to improve display performance of the liquid crystal by providing a planarization layer made of an organic insulating layer on an active matrix substrate is disclosed in JP-A-6-242433.
Also, a technique (color filter on TFT technique) of improving an aperture ratio by providing a color filter layer on an active matrix substrate is disclosed in JP-A-8-122824.
Further, a method (unevenness reflection plate formation technique) of manufacturing a good reflective type liquid crystal display device with small reflection by forming unevenness by an organic insulating layer on an active matrix substrate and providing a reflection electrode thereon is disclosed in JP-A-5-232465.
A manufacturing method of a TFT array by an organic interlayer separation technique will be described below as one example. Incidentally, a technique using polycrystalline silicon TFT as a switching element is disclosed in JP-A-6-242433, but here, a technique using a channel etching type a-Si TFT as a switching element will be described for consistency with the conventional art.
In the case of this TFT array, a planarization layer made of a thick film is provided on a passivation film 7c and further a transparent conductive film acting as a pixel electrode 6 is provided on the planarization layer. This transparent conductive film is connected to a drain electrode 4 via a contact through-hole 5 through the planarization layer and the passivation film 7c. 
Next, a manufacturing method of the TFT array by the organic interlayer separation technique will be described. A description to the fourth patterning (FIG. 4D) which is a formation step of a passivation film is omitted since the description is equal to that of the above-mentioned conventional art.
After the fourth patterning, the planarization layer is formed. Specifically, after a transparent photosensitive resist made of acrylic resin etc. is applied by a spin coat method, fifth patterning for opening the contact through-hole in the planarization layer by a photolithography step is performed.
Finally, as shown in FIG. 4E, the transparent conductive film acting as the pixel electrode 6 is formed by sputtering method and sixth photolithography step is performed.
However, in the manufacturing method of the TFT array by the organic interlayer separation technique, patterning steps increase by one step for forming the planarization layer. As a result of this, manufacturing steps become complicated to cause an increase in cost and a decrease in productivity cannot be avoided.
This is also similar to the color filter on TFT technique or the unevenness reflection plate formation technique, and by a step for forming a color filter layer or an overcoat layer, a step for forming an unevenness layer, respectively, the manufacturing steps become complicated and the productivity decreases.